1. Field of the Invention
The present invention relates to a polycarboxylic acid mixture. More particularly, the present invention is concerned with a polycarboxylic acid mixture comprising 80% by weight or more of 1,3,6-hexanetricarboxylic acid, wherein the polycarboxylic acid mixture has a psychometric lightness L-value of 98 or more, a psychometric chroma a-value of from −2.0 to 2.0 and a psychometric chroma b-value of from −2.0 to 3.0, and has a nitrogen content of 5,000 ppm by weight or less. The polycarboxylic acid mixture of the present invention has not only excellent color tone (i.e., not discolored and substantially colorless and transparent) but also excellent color tone stability under heating (hereinafter, this stability is frequently referred to simply as “heat stability”). Therefore, the polycarboxylic acid mixture of the present invention can be advantageously used for producing, for example, a paint, a detergent, a builder for a cleaning agent, an anti-limescale agent, a lubricating oil, and various polycarboxylic acid derivatives, such as esters. The present invention relates also to a method for producing the above-mentioned polycarboxylic acid mixture in high yield.
2. Prior Art
1,3,6-Hexanetricarboxylic acid is water-soluble and has good biodegradability. Therefore, recently, various application fields of 1,3,6-hexanetricarboxylic acid have been proposed.
For example, it has been reported that 1,3,6-hexanetricarboxylic acid which is derived from 1,3,6-tricyanohexane, as well as a salt of 1,3,6-hexanetricarboxylic acid, can be advantageously used as a raw material for preparing a detergent and as a lubricating agent for a chain belt for a toothed wheel (see Unexamined German Patent Application Laid-Open Specification No. 19637428).
1,3,6-Hexanetricarboxylic acid is a trifunctional acid; therefore, when it is used as a curing agent for, e.g., an epoxy compound, a cured product having a high crosslinkage density can be obtained. Especially, 1,3,6-hexanetricarboxylic acid can be advantageously used as a crosslinking agent for a paint.
Generally, 1,3,6-hexanetricarboxylic acid can be easily obtained by hydrolyzing 1,3,6-tricyanohexane. 1,3,6-Tricyanohexane can be obtained as a by-product in a process for producing adiponitrile from acrylonitrile by electrodimerization. 1,3,6-Tricyanohexane can also be produced by reacting acrylonitrile with adiponitrile in the presence of a base.
When an electrodimerization reaction of acrylonitrile is conducted, 1,3,6-tricyanohexane is obtained as a by-product and contained in a mixture of nitrites including adiponitrile as a main product. By removing the adiponitrile from the mixture by distillation or the like, the content of 1,3,6-tricyanohexane in the mixture can be increased.
However, the nitrile mixture comprised mainly of 1,3,6-tricyanohexane, which is obtained by the above-mentioned methods, is usually markedly discolored to assume a color of yellow or black.
For example, in Unexamined Japanese Patent Application Laid-Open Specification No. Sho 62-270550, 1,3,6-tricyanohexane is purified by molecular distillation, but the resultant purified product is a yellow liquid having a Hazen value of 400 or more.
A polycarboxylic acid mixture comprised mainly of 1,3,6-hexanetricarboxylic acid, which is conventionally obtained by hydrolyzing such a markedly discolored nitrile mixture as mentioned above, is also markedly discolored to assume a color of, for example, yellow brown or red brown. Further, such a discolored polycarboxylic acid mixture has a problem in that, when the mixture is heated, for example, at 80° C. or more, it exhibits an increased degree of discoloration; specifically, the color difference (ΔE) as between before and after the heating tends to be far greater than 1 and, hence, the color tone stability of the discolored polycarboxylic acid mixture under heating is poor. Therefore, a number of attempts have been made to obtain a polycarboxylic acid mixture which has an improved color tone by removing the undesired color from the discolored polycarboxylic acid mixture comprised mainly of 1,3,6-hexanetricarboxylic acid.
For example, Unexamined German Patent Application Laid-Open Specification No. 19637428 discloses the following method for producing a colorless or light yellow 1,3,6-hexanetricarboxylic acid. A nitrile mixture comprised mainly of 1,3,6-tricyanohexane, which is by-produced in an electrodimerization reaction of acrylonitrile, is hydrolyzed by heating, under reflux, the nitrile mixture together with a 20% aqueous sodium hydroxide solution to thereby obtain a hydrolysis reaction mixture, followed by cooling of the reaction mixture. A concentrated sulfuric acid is added to the obtained hydrolysis reaction mixture to thereby obtain a carboxylic acid mixture. The obtained carboxylic acid mixture is thoroughly dried to thereby obtain a beige residue. The beige residue is subjected to an extraction treatment with a dehydrated alkyl acetate by means of a Soxhlet's extractor, to obtain an extract, followed by removal of the alkyl acetate from the obtained extract under reduced pressure, thereby obtaining a colorless or light yellow 1,3,6-hexanetricarboxylic acid. Also, in this patent document, as a method other than the above-mentioned method using a Soxhlet's extractor, there is also described the following method for producing a colorless or light yellow 1,3,6-hexanetricarboxylic acid. The above-mentioned carboxylic acid mixture is extracted with tert-butyl methyl ether three times to thereby obtain an extract. Water is removed from the obtained extract by using magnesium sulfate, and the resultant is subjected to distilling off of the extraction solvent to thereby obtain a residue. The obtained residue is introduced into a mixture of acetone and cyclohexane, and the resultant mixture is cooled to thereby deposit crystals, followed by recovery of the crystals by filtration, thereby obtaining a colorless or light yellow 1,3,6-hexanetricarboxylic acid.
Further, in the above-mentioned Unexamined German Patent Application Laid-Open Specification No. 19637428, there is also described the following method for producing a colorless or light yellow 1,3,6-hexanetricarboxylic acid. 1,3,6-Tricyanohexane is mixed with crushed ice and a concentrated sulfuric acid, and the resultant mixture is subjected to hydrolysis at 140° C. to thereby obtain an aqueous mixture. The obtained aqueous mixture is extracted with tert-butyl methyl ether three times to thereby obtain an ether mixture. Water is removed from the obtained ether mixture by using magnesium sulfate, and the resultant is subjected to distilling off of the ether, thereby obtaining a colorless or light yellow 1,3,6-hexanetricarboxylic acid.
However, a 1,3,6-hexanetricarboxylic acid obtained by any of the above-mentioned methods described in Unexamined German Patent Application Laid-Open Specification No. 19637428 is disadvantageous in that it has a psychometric lightness L-value of less than 98 or a psychometric b-value of 3 or more and has poor heat stability such that the color difference (ΔE) as between before and after the 1,3,6-hexanetricarboxylic acid is allowed to stand for 18 hours at 80° C. is more than 2. (With respect to the above-mentioned color difference, explanations are given below.) Further, in any of the above-mentioned methods described in Unexamined German Patent Application Laid-Open Specification No. 19637428, it is necessary to use a large amount of an extraction solvent and, therefore, these methods are commercially disadvantageous.
Further, Agric. Biol. Chem., 45 (1) 57-62, 1981 discloses the following method for producing a colorless crystal of 1,3,6-hexanetricarboxylic acid. 1,3,6-Tricyanohexane is hydrolyzed with hydrochloric acid, and the resultant hydrolysis reaction mixture is allowed to stand at room temperature to thereby precipitate an ammonium salt (ammonium chloride), followed by filtration of the resultant to remove the ammonium salt and obtain a filtrate. The obtained filtrate is dried to obtain a crystal of 1,3,6-hexanetricarboxylic acid having a low purity. The obtained low purity crystal is dissolved in hot water, and the resultant solution is treated with activated carbon, followed by recrystallization, thereby obtaining a colorless crystal of 1,3,6-hexanetricarboxylic acid. The obtained 1,3,6-hexanetricarboxylic acid has a psychometric lightness L-value of 98 or more, a psychometric chroma a-value of 0.8 and a psychometric chroma b-value of 1.1, and is colorless. However, 1,3,6-hexanetricarboxylic acid produced by this method exhibits poor heat stability such that, when the 1,3,6-hexanetricarboxylic acid is heated, for example, at 160° C. for one hour or more, it is likely to suffer a marked discoloration. Further, the 1,3,6-hexanetricarboxylic acid produced by this method tends to disadvantageously contain a chlorine ion.